a+b=a+b/2+b/2,a=a+b/2-b/2sin(a+b)-sina=sin[（a+b/2）+(b/2)]-sin[（a+b/2）-(b/2)]展开=sin（a+b/2）*cosb/2+cos（a+b/2）*sinb/2-[sin（a+b/2）*cosb/2-cos（a+b/2）*sinb/2]=2sin（b/2）cos（a+b/2）...